This invention relates to semiconductor test equipment. More specifically, the invention relates to methods for determining errors in a semi-conductor probe card and probe machine.
A variety of equipment and techniques have been developed to assist manufacturers of integrated circuits for testing those circuits while still in the form of dies on semiconductor wafers. In order to quickly and selectively electrically interconnect metalized contact pads (also known as xe2x80x9cbonding padsxe2x80x9d) on each die to electrical test equipment (known as a xe2x80x9cprober machinexe2x80x9d), arrays of slender wires or other contact media are provided. The contact media are arranged on conventional printed circuit boards so as to be positionable on the metalized contact pads associated with each semiconductor die. As is well known by those of ordinary skill in the art, those printed circuit board test cards have come to be known as xe2x80x9cprobe cardsxe2x80x9d or xe2x80x9cprobe array cardsxe2x80x9d, and the contact media have come to be known as xe2x80x9cprobe card pinsxe2x80x9d or xe2x80x9cprobe pinsxe2x80x9d or xe2x80x9cprobe wiresxe2x80x9d.
As the component density of semiconductor circuits has increased, the number of contact pads associated with each die has increased. It is now not uncommon for a single die to have upwards of 600 pads electrically associated with each die. The metalized pads themselves may have as little as a ten xcexcm gap therebetween with an on-center spacing on the order of 50 xcexcm to 100 xcexcm. As a result, the slender probe wires of the probe array cards have become much more densely packed. It is highly desirable that the free ends or xe2x80x9ctipsxe2x80x9d of the probes be aligned in a common horizontal plane, as well as have the proper positioning with respect to one another within the plane so that when the probes are pressed down onto the metalized pads of an integrated circuit die by a prober machine, the probes touch down substantially simultaneously, and with equal force while being on target. As used herein, the terms xe2x80x9ctouchdownxe2x80x9d, xe2x80x9crestxe2x80x9d and xe2x80x9cfirst contactxe2x80x9d have the same meaning. In the process of making electrical contact with the pads, the probes are xe2x80x9cover traveledxe2x80x9d causing the probes to deflect from their rest position. This movement is termed xe2x80x9cscrubxe2x80x9d and must be taken into account in determining whether the rest position and the over travel position of the probes are within specification for the probe card.
The assignee of the present invention has developed equipment for testing the electrical characteristics, planarity and horizontal alignment, as well as scrub characteristics of various probe cards and sells such equipment under its PRECISION POINT(trademark) line of probe card array testing and rework stations. A significant component of these stations is a planar working surface known as a xe2x80x9ccheckplatexe2x80x9d. A check plate simulates the semi-conductor die undergoing a test by a probe card while checking the above described characteristics of the probes. A suitable check plate for use with the assignee""s PRECISION POINT(trademark) equipment is described in detail in U.S. Pat. No. 4,918,374 to Stewart et al. issued Apr. 17, 1990, the disclosure of which is incorporated herein by reference. It is sufficient for the purposes of this disclosure to reiterate that while the subject probe card is held in a fixed position the check plate is moved horizontally in steps when testing the horizontal relative positioning, and vertically in steps when testing the touchdown contact and over travel position of each probe tip. Previously, and as described in the above-identified patent, horizontal position information for each probe tip was determined by translating an isolated probe tip in steps across resistive discontinuities on the check plate. In recent years, this technique has been altered by placing a transparent, optical window in the surface contact plane of the check plate with a sufficiently large surface dimension so as to permit a probe tip to reside thereon. An electronic camera viewing the probe tip through the window digitizes the initial touch down image of the probe, and a displaced position of the probes due to xe2x80x9cscrubxe2x80x9d as the check plate is raised to xe2x80x9cover travelxe2x80x9d the probe. The initial touch down position is compared to the anticipated touch down position to assist an operator in realigning that particular probe.
Another prior art technique for determining relative probe tip positions in a horizontal (e.g. X-Y) plane is described in U.S. Pat. No. 5,657,394 to Schwartz et al., the disclosure of which is incorporated herein by reference. The system disclosed therein employs a precision movement stage for positioning a video camera into a known position for viewing probe points through an optical window. Analysis of the video image and the stage position information are used to determine the relative positions of the probe points. In systems of this type, a xe2x80x9creferencexe2x80x9d probe position is determined primarily through information from the video camera, combined with position information from the precision stage. If the pitch of the probes on the probe card is small enough, two or more probes can be simultaneously imaged with the video camera. The position of this adjacent probe is then referenced with respect to the xe2x80x9creferencexe2x80x9d probe from information from the video camera only. The camera is then moved to a third probe, adjacent to the second probe and this process is repeated until each probe on the entire probe card has been imaged.
In addition to the above devices for measuring various parameters of probe cards, equipment is available for measuring actual xe2x80x9cscrub marksxe2x80x9d made by probe card pins on a test wafer which has been impressed by the probe card with a prober machine. One such apparatus is manufactured by Visioneering Research Laboratory, Inc., Las Cruces, N. Mex. to provide high quality imaging of scrub marks made by a probe card and a prober machine. It is well known that scrub patterns analyzed by a probe card analysis (hereinafter xe2x80x9cPCAxe2x80x9d) machine do not match the scrub marks produced on a test wafer imaged by a scrub mark analysis (hereinafter xe2x80x9cSMAxe2x80x9d) machine. The test wafer models the surface characteristics of bonding pads on a semiconductor die.
As stated above, the measurement surface on the probe card analyzer is typically manufactured from hardened steel, or more recently a transparent synthetic or natural crystal such as sapphire. This PCA testing surface is much harder than the aluminized surface of a semiconductor bonding pad. The typical annealed aluminum surface of a semi-conductor bonding pad in fact yields under pressures applied by the semiconductor probing machine which may be on the order of 5 grams per pin. Remembering that the pin surface is very small, the pressure applied is sufficient to break the surface of the aluminum bonding pad causing the probe tip to xe2x80x9cdig inxe2x80x9d during probe pin overtravel. Within a short distance, the tip of the probe pin plows so deeply into the aluminum surface that it stops even though the probe card continues its downward travel. This phenomenon has been characterized as xe2x80x9cstubbingxe2x80x9d by the assignee of the present invention. In contrast, the hard metal or sapphire surface of the probe card analysis machine does not yield under pressure from the probe pin. In addition, the metal or sapphire contact surface of the probe card analysis machine is highly polished and has a much lower coefficient of friction than does the aluminized surface of the semiconductor die bonding pad. As a result, the probe pin does not xe2x80x9cstubxe2x80x9d on the probe card analysis machine, and the probe pin tip travels further than it does on the aluminized bonding pad. Furthermore, the place at which the probe pin first contacts an aluminized bonding pad (or the aluminized semiconductor test wafer which simulates the bonding pad in the scrub mark analysis machine) or xe2x80x9ctouch downxe2x80x9d position of the probe pin is not readily discernable in the scrub mark made in the aluminum surface. The scrub mark resembles a brush stroke with a faint starting position and a deep, clearly defined ending position. Conversely, the probe card analysis machine accurately captures the touch down position of the probe pin on the measuring surface as well as its full travel across the surface without stubbing. Therefore, neither the touch down position, nor the end of travel position of the probe pin on the probe card analysis machine, matches corresponding positions on either an actual aluminum bonding pad or on a semiconductor test wafer imaged by a scrub mark analysis machine.
As is well known by those of ordinary skill in the art, it is desirable to accurately model the trajectory of a probe pin on a semiconductor bonding pad through the use of analysis and test equipment. It is further apparent that neither the probe card analysis machine, nor the scrub mark analysis machine alone provide accurate data as to the true touch down position of a probe on a metalized bonding pad, and the true end of travel position of the probe pin on an aluminized bonding pad.
Therefore, a need exists for a measurement and analysis technique which will accurately predict the behavior of a semiconductor probe card pin on an aluminized bonding pad.
A further need exists for a probe card analysis and measurement technique which will quantitatively determine when a probe card and prober machine combination are out of tolerance for a specified task.
It is therefore an object of the present invention to provide a method for accurately predicting the behavior of a semiconductor probe card probe pin on a semiconductor bonding pad.
It is further an object of the present invention to achieve the above object while quantitatively providing data relating to the adequacy of a probe card and prober machine combination in performing a task accurately.
It is yet another object of the present invention to provide data for optimizing performance of a probe card and/or prober machine based on the predicted behavior of a semiconductor probe card pin on a semiconductor die bonding pad.
The invention achieves these objects, and other objects and advantages which will become apparent from the description which follows by measuring the scrub pattern of semiconductor probe card pins with a probe card analysis machine, measuring scrub marks on a semiconductor test wafer made by a prober machine with a probe card of interest, and merging the resulting data to provide a data set having predictably accurate touch down and end of travel data for a plurality of probe card probe pins on a metalized semiconductor die surface.
In a preferred embodiment of the invention, error values are assigned to different, corresponding measurements in a data set from the probe card analysis machine and a data set from the scrub mark analysis machine. The error values are then minimized by iteratively incrementing mathematical horizontal, vertical and rotational values (e.g. X, Y and 0) until the differences between the corresponding data from the scrub mark analysis machine and probe card analysis machine are minimized.